Many industrial processes result in the emission of small hazardous particles into the atmosphere. These particles often include very fine sub-micron particles of toxic compounds which are easily inhaled. Their combination of toxicity and ease of respiration has prompted governments around the world to enact legislation for more stringent control of emission of particles less than ten microns in diameter (PM10), and particularly particles less than 2.5 microns (PM2.5).
Smaller particles in atmospheric emissions are also predominantly responsible for the adverse visual effects of air pollution. Opacity is largely determined by the fine particulate fraction of the emission since the light extinction coefficient peaks near the wavelength of light which is between 0.1 and 1 microns.
Various methods have been used to remove dust and other pollutant particles from air streams. Although these methods are generally suitable for removing larger particles from air streams, they are usually much less effective in filtering out smaller particles, particularly PM2.5 particles.
Fine particles in air streams can be made to agglomerate into larger particles by collision/adhesion, thereby facilitating subsequent removal of the particles by filtration. Our international patent applications nos. PCT/NZ00/00223 and PCT/AU2004/000546 disclose energized and passive devices for agglomerating particles. The agglomeration efficiency is dependent upon the incidence or frequency of collisions and similar interactions between the particles.
Many pollution control strategies also rely on contact between individual elements of specific species to promote a reaction or interaction beneficial to the subsequent removal of the pollutant concerned. For example, sorbents such as activated carbon can be injected into the polluted air stream to remove mercury (adsorption), or calcium can be injected to remove sulfur dioxide (chemisorption).
In order for these interactions to take place, the two species of interest must be brought into contact. For many industrial pollutants in standard flue ducts, this is difficult for several reasons. For example, the time frames for reaction/interaction are short (of the order of 0.5-1 second), the species of interest are spread very sparsely (relative to the bulk fluid) through the exhaust gases, and the scale of the flue ducting is large compared to the scale of the pollutant particles.
Normally, exhaust gases from the outlet of an industrial process are fed into a large duct which transports them to some downstream collection device (e.g. an electrostatic precipitator, bag filter, or cyclone collector) as uniformly and with as little turbulence/energy loss as possible. Such turbulence as is generated en route is normally a large scale diversion of gases around turning vanes, around internal duct supports/stiffeners, through diffusion screens and the like. This turbulence is of the scale of the duct and should desirably be the minimum disturbance, and hence pressure drop, possible to achieve the desired flow correction.
Similarly, when mixing devices are employed for a specific application, eg. sorption of a particular pollutant, they are usually devices that generate a large-scale turbulence field (of the order of the duct width or height) and are arranged as a short series of curtains that the gases must pass through.
The aim of most known mixing devices is to achieve a homogeneous mixture of two or more substances. Such devices are not specifically designed to promote interactions between fine particles in the mixture. In most industrial-scale devices involving the transport of particles, the turbulence generated by the mixing is of a large scale relative to the particles. Under such conditions the particles tend to travel in similar paths rather than in collision courses.
It is also known that vortex generators can be used in mixing chambers to promote mixing of fluids. However such devices are not generally used in particle laden flows to create collisions between particles.
Whether they be particulate (e.g. flyash), gaseous (e.g. SO2), mist (e.g. NOx), or elemental (eg. Mercury), the pollution species which are the more difficult to collect within industrial exhaust flues are those of the order of micrometers in diameter (i.e. 10−6 metres). Due to their small size, they occupy a very small volumetric proportion of the total fluid flow. For example, if uniformly distributed, one million 1 μm diameter particles would occupy less than 0.00005% of the volume of 1 cm3 of gas (assuming that the particles are spherical). Even at 10 μm diameter, this proportion only increases to 0.05%. When it is considered that a pollutant such as Mercury may only account for a few parts per million (ppm) of the total species present, it is apparent that at particle scale, there is a significant amount of space/distance between the species being transported by an industrial flue gas. Where particles are already “well-mixed” in a flow, e.g. disbursed more-or-less randomly throughout a duct (as in an exhaust flue), turbulence of any scale will not be able to mix them more thoroughly.
Furthermore, sufficiently small particles that are entrained in a flowing fluid will follow the streamlines in the fluid flow. This occurs where the viscous forces of the fluid dominate the inertial forces of the particle. Known turbulent mixing regimes of the scale of the duct are many orders of magnitude larger than the particle. When viewed from the perspective of the particle, they are far from being chaotic but rather, are relatively smooth. Whilst there may be many changes of direction for a particle in its passage through a turbulent flow in a duct or through a standard mixing region, they are all relatively long range compared with the size or scale of the particle. Consequently, particles in a stream under conditions typical of industrial dust-laden flows follow more or less the same paths as their neighbouring particles, resulting in few interactions with the surrounding particles. At particle scale therefore, there are relatively few turbulence-generated interactions, and consequently, the known mixing processes achieve poor efficiency in agglomeration.
Systems intended to maximise the collision rate of very small pollution species which occupy a tiny proportion of the volume of the total fluid flow must cause them to move along different trajectories, and/or at different speeds, to each other, as often as possible. Additionally such differences in trajectory and/or speed must be brought to bear at the scale of the particle to have the most effect. Unfortunately, current design philosophies do not adequately address these criteria.
It is an aim of the present invention to provide method and apparatus for achieving improved interaction of particles in fluid flows.
It is another aim of this invention to provide a method of custom designing a formation to generate particle scale turbulence to cause interactions between particular types of particles in a fluid flow in a highly efficient manner.